Micro-electro-mechanical systems (MEMS) apparatuses and methods are presently being developed for a wide variety of applications in view of the size, cost and power consumption advantages provided by these devices. Specifically, a variable capacitor, also known as a varactor, can be fabricated utilizing MEMS technology. Typically, a variable capacitor includes an interelectrode spacing (or an electrode overlap area) between a pair of electrodes that can be controllably varied in order to selectively vary the capacitance between the electrodes. In this regard, conventional MEMS variable capacitors include a pair of electrodes, one that is typically disposed upon and fixed to the substrate and the other that is typically carried on a movable actuator or driver. In accordance with MEMS technology, the movable actuator is typically formed by micromachining the substrate such that very small and very precisely defined actuators can be constructed.
As appreciated by persons skilled in the art, many types of MEMS variable capacitors and related devices can be fabricated by either bulk or surface micromachining techniques. Bulk micromachining generally involves sculpting one or more sides of a substrate to form desired three dimensional structures and devices in the same substrate material. The substrate is composed of a material that is readily available in bulk form, and thus ordinarily is silicon or glass. Wet and/or dry etching techniques are employed in association with etch masks and etch stops to form the microstructures. Etching is typically performed through the backside of the substrate. The etching technique can generally be either isotropic or anisotropic in nature. Isotropic etching is insensitive to the crystal orientation of the planes of the material being etched (e.g., the etching of silicon by using a nitric acid as the etchant). Anisotropic etchants, such as potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH), and ethylenediamine pyrochatechol (EDP), selectively attack different crystallographic orientations at different rates, and thus can be used to define relatively accurate sidewalls in the etch pits being created. Etch masks and etch stops are used to prevent predetermined regions of the substrate from being etched.
On the other hand, surface micromachining generally involves forming three-dimensional structures by depositing a number of different thin films on the top of a silicon wafer, but without sculpting the wafer itself. The films usually serve as either structural or sacrificial layers. Structural layers are frequently composed of polysilicon, silicon nitride, silicon dioxide, silicon carbide, or aluminum. Sacrificial layers are frequently composed of polysilicon, photoresist material, polimide, metals, or various types of oxides, such as PSG (phosphosilicate glass) and LTO (low-temperautre oxide). Successive deposition, etching, and patterning procedures are carried out to arrive at the desired microstructure. In a typical surface micromachining process, a silicon substrate is coated with an isolation layer, and a sacrificial layer is deposited on the coated substrate. Windows are opened in the sacrificial layer, and a structural layer is then deposited and etched. The sacrificial layer is then selectively etched to form a free-standing, movable microstructure such as a beam or a cantilever out of the structural layer. The microstructure is ordinarily anchored to the silicon substrate, and can be designed to be movable in response to an input from an appropriate actuating mechanism.
MEMS variable capacitors have been fabricated that include a movable, capacitive plate (or electrode) that is suspended above first and second coplanar electrodes. The variable capacitor operates by applying a voltage across the first electrode and the movable plate so that the plate is deflected towards the first electrode by electrostatic attraction. As the movable plate moves, the spacing between the second electrode and the movable plate changes, thus changing the capacitance value between the second electrode and the plate. A signal line is usually connected to the second electrode and the plate to sense the change in capacitance for use in various Radio Frequency functions. One problem with this configuration is that the voltage supply is electrically connected to the signal line through the plate that can result in undesirable noise/interference or degradation of the signal on the signal line. Thus, this configuration may require additional components to combine/separate the signal and actuation voltage, leading to a more complex and costly implementation. Another problem is that the RF voltage exerts an equivalent force on the movable plate to that exerted by the intended control voltage, leading to control complexity and increased intermodulation.
Other known MEMS variable capacitors provide parallel-plate electrodes that move linearly. The electrodes of these variable capacitors are subject to suddenly “snapping down” towards one another after moving close enough to one another. These types of variable capacitors are also subject to microphonics and stiction problems.
Some MEMS variable capacitors are based upon electro-thermally actuated parallel-plate design. These types of variable capacitors are subject to reduced power handling capability due to gap reduction and the likelihood for breakdown occurrence. These variable capacitors also consume excessive power, especially if the electro-thermal actuation must be applied continuously to maintain the capacitance value.
Other MEMS variable capacitors utilize a massively-parallel, interdigited-comb device for actuation. These variable capacitors are so sensitive to parasitic substrate capacitance that they require either a high-resistivity substrate such as glass or the removal of the substrate beneath the MEMS device. Thus, this type of variable capacitor is not readily integrated into a conventional integrated circuit (IC) process. Additionally, the MEMS device is physically large because the capacitance dependence on the overlap of comb fingers requires large aspect ratios. These devices require excessive space and cause a low resonant frequency resulting in shock and vibration problems.
Therefore, it is desirable to provide novel variable capacitor apparatuses and related methods for MEMS applications that improve upon aforementioned designs.